Inorganic solid electrolyte and lithium cell component

ABSTRACT

A sulfide-based inorganic solid electrolyte that suppresses the reaction between silicon sulfide and metallic lithium even when the electrolyte is in contact with metallic lithium, a method of forming the electrolyte, and a lithium battery&#39;s member and lithium secondary battery both incorporating the electrolyte. The electrolyte comprises Li, P, and S without containing Si. It is desirable that the oxygen content vary gradually from the electrolyte to the lithium-containing material at the boundary zone between the two members when analyzed by using an XPS having an analyzing chamber capable of maintaining a super-high vacuum less than 1.33×10 −9  h Pa and that the oxygen-containing layer on the surface of the lithium-containing material be removed nearly completely. The electrolyte can be formed such that at least part of the forming step is performed concurrently with the step for etching the surface of the substrate by irradiating the surface with inert-gas ions.

TECHNICAL FIELD

The present invention relates to an inorganic solid electrolyte and amethod of forming the electrolyte. The present invention also relates toa lithium battery's member and a lithium secondary battery bothincorporating the electrolyte.

BACKGROUND ART

Lithium secondary batteries incorporating organic electrolysis solutionshave been widely used. They are advantageous in that they have highenergy output per unit volume or unit weight in comparison with otherbatteries. In exploiting this advantage, researchers and engineers havebeen advancing the development for practical applications of the lithiumsecondary batteries as power sources for mobile communications devices,notebook-type personal computers, and electric cars.

To improve the performance of a lithium secondary battery, attempts havebeen made to use metallic lithium as the negative electrode. However,the repetition of charge and discharge causes dendritic metallic lithiumto grow on the surface of the negative electrode. This may lead to aninternal short circuit between the negative and positive electrodes,ultimately triggering explosion. To suppress the possibility of thisdangerous situation, engineers have studied the formation of a thinsulfide-based inorganic solid electrolytic layer on the metalliclithium. An example of this study has been disclosed in the publishedJapanese patent application Tokukai2000-340257.

The technology on the solid electrolyte for lithium batteries and otherapplications has been disclosed, for example, in “Solid State Ionics, 5(1981) 663–666,” “DENKI KAGAKU(Japanese expression meaningelectrochemistry) 65, No. 11 (1997) 914–919,” the published Japanesepatent application Tokukai 2001-250580, “J. Am. Ceram. Soc., 84 [2]477–79 (2001),” the published Japanese patent application Tokukaihei5-48582, the published Japanese patent application Tokukaihei 4-231346,“The 26th Symposium on Solid State Ionics in Japan, Extended Abstracts(2000) 174–175,” U.S. Pat. No. 6,025,094, and U.S. Pat. No. 5,314,765.

On the other hand, it has been revealed that when a thin sulfide-basedinorganic solid electrolytic layer containing silicon sulfide is incontact with metallic lithium, the silicon in the silicon sulfide (SiS₂)is reduced by the metallic lithium, and consequently the inorganic solidelectrolyte degrades with time even at room temperature.

Generally, a compound layer having low ionic conductivity, such as anoxide layer, is formed on the surface of metallic lithium. When theoxide layer is formed, the reaction between the metallic lithium and thesilicon sulfide tends to be suppressed. Therefore, if the oxide layer isremoved to improve the performance of the battery, the inorganic solidelectrolyte degrades distinctly with time due to the reaction betweenthe metallic lithium and the silicon sulfide.

DISCLOSURE OF INVENTION

A principal object of the present invention is to offer (a) asulfide-based inorganic solid electrolyte that suppresses the reactionbetween silicon sulfide and metallic lithium even when the electrolyteis in contact with metallic lithium and (b) a method of forming theelectrolyte. Another principal object of the present invention is tooffer a battery's member and a lithium secondary battery bothincorporating the electrolyte.

The present invention is based on the findings that the metallic lithiumreacts with the silicon sulfide (SiS₂) at room temperature, degradingthe inorganic solid electrolyte. The present invention attains theforegoing object by offering an inorganic solid electrolyte containingno Si.

<Inorganic Solid Electrolyte>

According to the present invention, the inorganic solid electrolytecomprises Li, P, and S without containing Si. In particular, it isdesirable that the electrolyte comprise:

-   -   (a) at least 20 atom. % and at most 60 atom. % Li; and    -   (b) the remainder comprising P and S without containing Si.        It is more desirable that the electrolyte comprise:    -   (a) at least 25 atom. % and at most 60 atom. % Li; and    -   (b) the remainder comprising P and S without containing Si.        The expression “to comprise elements A, B, and so on ”is used        throughout Description, Claims, and Abstract to mean “to        comprise elements A, B, and so on together with unavoidable        impurities.”

It is desirable that Si not be contained in the vitreous framework ofthe inorganic solid electrolyte. It was generally accepted that Si wasinevitably contained in a thin sulfide-based inorganic solidelectrolytic layer. However, it was not known that the Si adverselyaffected the performance of the inorganic solid electrolyte.Consequently, no countermeasures were taken. On the other hand, thepresent invention eliminates the Si from the inorganic solid electrolyteto suppress the electrolyte from degrading with time due to the reactionbetween the metallic lithium and the silicon sulfide. In the abovedescription, the term “to eliminate the Si” is used to mean “to reducethe Si content to less than 1 atom. %.”

A lithium-phosphorus-sulfur-based inorganic solid electrolyte containingno silicon is superior to other sulfide-based inorganic solidelectrolytes, particularly sulfide-based inorganic solid electrolytescontaining silicon, because it has the following advantages:

-   -   1. It has high ionic conductivity.    -   2. Unlike a sulfide-based inorganic solid electrolyte containing        silicon, even when placed in contact with metallic lithium, it        does not react with the metallic lithium.    -   3. When a negative electrode is produced by forming it on        metallic lithium for use in a lithium secondary battery having a        nonaqueous organic electrolysis solution, it has the following        properties:        -   (a) It is excellent in suppressing the growth of dendrites.        -   (b) It is excellent in chemical stability against the            electrolysis solution.        -   (c) It has high mechanical strength against the morphology            shift during charge-and-discharge cycles.    -   4. As opposed to costly SiS₂, phosphorus sulfide (P₂S₅) is        comparatively low-cost, easily available, and therefore suitable        for industrial production.

The present invention specifies that the content of the element lithiumin the inorganic solid electrolytic layer be at least 20 atom. % and atmost 60 atom. %. If the content is less than 20 atom. %, the ionicconductivity is decreased, and the inorganic solid electrolytic layercomes to have high resistance. In addition, the bonding strength betweenthe electrolytic layer and the metallic-lithium layer decreases. On theother hand, if the content is more than 60 atom. %, although the bondingstrength between the electrolytic layer and the metallic-lithium layerincreases, the electrolytic layer is polycrystallized and becomesporous, making it difficult to form a dense continuous layer of aninorganic solid electrolyte. Furthermore, the electrolytic layer comesto have electronic conductivity and thereby causes internalshort-circuiting when used to produce a battery, decreasing theperformance of the battery. In other words, it is desirable that theelectrolytic layer be amorphous. Throughout Description and Claims, theexpression “the electrolytic layer or electrolyte is amorphous” is usedto mean that “the electrolytic layer or electrolyte is made of asubstantially amorphous (vitreous) material.” More specifically, theforegoing amorphous state includes the following cases when examined bythe X-ray diffraction method:

-   -   (a) A halo pattern is observed.    -   (b) In addition to a halo pattern, tiny peaks are observed due        to the starting materials and reaction products (this phenomenon        is observed when crystal grains are partly produced from the        starting materials and reaction products).    -   (c) Peaks are observed due to the substrate for the thin-layer        formation.

It is desirable that the content of the element phosphorus be at least 3atom. % and at most 20 atom. %. It is desirable that the content of theelement sulfur be at least 30 atom. % and at most 60 atom. %. If thecontents of phosphorus and sulfur are insufficient, defects tend tooccur. If the contents of phosphorus and sulfur are excessive, theamounts of free phosphorus and free sulfur increase as impurities in thesolid electrolyte.

It is desirable that a specific compound to be contained in theinorganic solid electrolyte of the present invention be a compound oflithium sulfide (Li₂S) and phosphorus sulfide (P₂S₅). In particular, itis desirable that the ratio X/Y be at least 1.0 and at most 19, where Xdenotes the ratio of the constituent lithium (Li), and Y denotes theratio of the constituent phosphorus (P).

The inorganic solid electrolyte may contain oxygen, nitrogen, or both.When the inorganic solid electrolyte contains about 5 atom. % or lessoxygen or nitrogen, it can exhibit high lithium-ion conductivity. Thisis attributed to the fact that the small amounts of oxygen or nitrogenatoms widen the interstices of the formed amorphous framework, reducingthe interference to the movement of the lithium ions.

The types of compounds containing oxygen include Li₃PO₄, Li₂SO₄, Li₂O,and P₂O₅. The types of compounds containing nitrogen includeLi₃PO_(4-x)N_(2x/3) (0<X<4).

It is desirable that the inorganic solid electrolyte be in the shape ofa thin layer. It is desirable that the thin layer have a thickness of atleast 0.01 μm and at most 10 μm, more desirably at least 0.05 μm and atmost 1 μm. If the thin layer is excessively thin, defects such aspinholes increase. If the thin layer is excessively thick, it requiresprolonged processing time, increasing the production cost.

<Member of Lithium Battery>

It is desirable that the foregoing inorganic solid electrolyte be formedon the surface of metallic lithium or a lithium alloy(lithium-containing material) so that it can be used as a member of abattery. The types of lithium alloys include alloys of Li and anotherelement such as In, Ti, Zn, Bi, and Sn.

A thin metal layer made of a metal that forms an alloy or intermetalliccompound with lithium, such as Al, In, Bi, Zn, or Pb, may be formed onthe surface of the lithium-containing material. When the thin metallayer and the lithium-containing material are used to constitute anegative electrode, the metallic lithium travels smoothly at the time ofcharge and discharge, and the utilizing thickness for the metalliclithium increases. In addition, this structure can equalize thedimensional change in the negative electrode at the time of charge anddischarge, reducing the strain given to the electrolytic layer.

The foregoing lithium-containing material may be used without anypre-treatment when the electrolytic layer is formed. However, thesurface of metallic lithium is generally covered by thin layers such asan oxide (such as Li₂₀O) layer, a carbonate (Li₂CO₃) layer, and ahydroxide (LiOH) layer. These layers have low lithium-ion conductivity,and therefore it is desirable to remove them. They can be removed byirradiating inert-gas-ion beams such as argon-ion beams. It is desirablethat the argon gas for this purpose have extremely high purity. Forexample, it is desirable to use an argon gas having a purity of99.9999%.

This pretreatment enables the formation of the thin inorganic solidelectrolytic layer directly on the lithium-containing material, therebyreducing the impedance between them.

In particular, it is desirable to perform at least part of the step forforming the thin inorganic solid electrolytic layer concurrently withthe step for etching the surface of the substrate by irradiating thesurface with inert-gas ions. This thin-layer-forming method enables theproduction of a lithium battery member in which:

-   -   (a) the oxygen content varies gradually from the inorganic solid        electrolyte to the lithium-containing material at the boundary        zone between the electrolyte and the lithium-containing material        when analyzed by using an X-ray photo-electronic spectroscope        (XPS) equipped with an analyzing chamber capable of maintaining        a super-high vacuum less than 1.33×10⁻⁹ hPa (1×10⁻⁹ Torr); and    -   (b) the oxide layer on the surface of the lithium-containing        material is removed nearly completely.        An example of the foregoing XPS is Type 5400 produced by        Physical Electronics, Inc.

When a thin electrolytic layer is formed after finishing the etchingstep (this is the conventional method) by using an ordinary vacuum unithaving an attainable vacuum degree of about 1.3×10⁻⁷ hPa (1×10⁻⁷ Torr),an oxygen-containing layer, such as an oxide layer, a carbonate layer,and a hydroxide layer, tends to be formed between the lithium-containingmaterial and the inorganic solid electrolyte. According to the presentinvention, on the other hand, at least part of the step for forming thethin electrolytic layer is performed concurrently with the etching step.This method enables the nearly complete elimination of theoxygen-containing layer even with the ordinary vacuum unit.

Although the etching step and the entire step for forming the thinelectrolytic layer may be carried out concurrently, it is desirable tostart the step for forming the thin electrolytic layer a short timebefore the etching step finishes. In other words, after the etching stepnearly completely removes the oxygen-containing layer on the surface ofthe substrate, the formation of the thin electrolytic layer starts underthe condition that prohibits the formation of an oxygen-containinglayer. In this case, the thin electrolytic layer is formed while part ofit is removed by the inert-gas ions.

When the etching step is carried out separately, the time period for theetching is determined by estimating the time for nearly completelyremoving the oxygen-containing layer on the surface of the substrate.When the step for forming the thin electrolytic layer starts a shorttime before the etching step finishes, the time period for theoverlapping is determined by estimating the time for covering the entiresurface of the substrate with the material of the thin electrolyticlayer.

There is an alternative method. After the nearly complete removal of theoxygen-containing layer, a nitride layer such as a lithium nitride(Li₃N) layer may be formed before the thin electrolytic layer is formed.Li₃N has an ionic conductivity of 1×10⁻³ S/cm or more. Therefore, thepresence of the Li₃N layer between the metallic lithium and the thininorganic solid electrolytic layer does not reduce the allowable currentdensity. The nitride layer can be expected to suppress the formation ofother compound layers such as an oxide layer.

The nitride layer can be formed, for example, by treating thelithium-containing material with argon-nitrogen-mixed-ion beams. Thenitride layer has only to be formed with a minimal thickness on thesurface of the lithium-containing material without leaving unnitridedspots. It is desirable that the nitride layer have a thickness of atleast 1 nm. On the other hand, if the nitride layer is excessivelythick, it creates various problems such as a reduction in ionicconductivity due to polycrystallization, a reaction with theelectrolysis solution, and a reduction in voltage-withstanding strength.Consequently, it is desirable that the nitride layer have a thickness ofat most 0.1 μm (100 nm), more desirably at most 10 nm.

<Lithium Secondary Battery>

The foregoing battery member can be used as a negative electrode of alithium secondary battery. For example, a lithium secondary battery canbe produced by (a) laminating a positive electrode, a porous separator,and a negative electrode, (b) impregnating the laminated body with anonaqueous electrolysis solution, and (c) housing the laminated body ina battery case to seal it. The above process is more specificallyexplained below. First, the negative electrode is bonded with anegative-electrode-side collector. A thin inorganic solid electrolyticlayer containing no organic electrolysis solution is formed on alithium-containing material used as the negative electrode. Thus, abonded body of the negative electrode and the thin electrolytic layer isproduced. Second, a material containing an organic high polymer isformed on a positive-electrode-side collector (copper or aluminum foil,for example) to obtain the positive electrode. Finally, the foregoingbonded body is coupled with the positive electrode through the separatorto produce the lithium secondary battery. This structure enables thereduction in the contact resistance between the negative electrode andthe electrolytic layer and between the positive electrode and theelectrolytic layer, attaining a good charge-and-discharge performance.In addition to the button-type battery in which the components arelaminated in the above-described manner, a cylindrical battery may beproduced by rolling up a laminated body of the negative electrode, theelectrolytic layer, and the positive electrode.

The separator to be placed between the positive electrode and the solidelectrolytic layer must be made of a material that has pores in whichlithium ions can travel and that is stable without dissolving in anorganic electrolysis solution. The separator may be produced by using,for instance, a nonwoven fabric or porous body formed of a material suchas polypropylene, polyethylene, fluororesin, or polyamide resin. A metaloxide film having pores may also be used.

<Method of Forming Inorganic Solid Electrolyte>

According to the present invention, an inorganic solid electrolyte isformed by forming a thin inorganic solid electrolytic layer on asubstrate. More specifically, a thin inorganic solid electrolytic layercomprising Li, P, and S without containing Si is formed while thesubstrate is heated at a temperature of at lowest 40° C. and at highest180° C. Alternatively, after the same electrolytic layer as describedabove is formed on a substrate at a temperature lower than 40° C., thesubstrate having the formed electrolytic layer is heated at atemperature of at lowest 40° C. and at highest 180° C. It is desirablethat the inorganic solid electrolyte comprise:

-   -   (a) at least 20 atom. % and at most 60 atom. % Li; and    -   (b) the remainder comprising P and S.        It is more desirable that the inorganic solid electrolyte        comprise:    -   (a) at least 25 atom. % and at most 60 atom. % Li; and    -   (b) the remainder comprising P and S.        As described above, a thin inorganic solid electrolytic layer        having high ionic conductivity can be obtained by heating the        substrate while the electrolytic layer is formed or by heating        the electrolytic layer after it is formed at room temperature.        On the other hand, if an inorganic solid electrolyte containing        Si is formed on the surface of a lithium-containing material        while the substrate is heated or if the same electrolyte is        heated after it is formed, the reaction between the Li and the        Si is undesirably promoted. As distinguished from this case, the        present invention specifies the method in which an inorganic        solid electrolyte containing no Si is formed on the surface of a        lithium-containing material. The above-described heat treatment        on this electrolyte of the present invention can give the        electrolyte good properties such as high ionic conductivity.        Therefore, this heat treatment is particularly effective.        Furthermore, when a thin inorganic solid electrolytic layer is        formed while the substrate is heated, other effects such as the        increase in the bonding strength between the thin electrolytic        layer and the substrate can be expected.

It is desirable that the foregoing thin inorganic solid electrolyticlayer be formed by any of the sputtering method, the vacuum evaporationmethod, the laser abrasion method, and the ion plating method.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of the present invention are explained below.

EXAMPLE 1

A thin metallic-lithium layer having a thickness of 10 μm was formed bythe vacuum evaporation method on a sheet of copper foil having athickness of 10 μm, a length of 100 mm, and a width of 50 mm. A thininorganic solid electrolytic layer was formed on the surface of the thinmetallic-lithium layer. In place of the thin metallic-lithium layer, asheet of metallic-lithium foil having the same length and width as thoseof the sheet of copper foil and a thickness of 30 μm may be laminatedwith the sheet of copper foil. In this case, a thin inorganic solidelectrolytic layer is formed on the sheet of metallic-lithium foil.

The process of the foregoing formation of the thin electrolytic layer isexplained in detail below. The substrate having the formed thinmetallic-lithium layer was placed in a vacuum thin-layer-forming unit.The attainable vacuum degree of the unit was 5.3×10⁻⁷ hPa (4×10⁻⁷ Torr).First, while an argon gas having a purity of 99.9999% was fed into theunit such that a gas pressure of 2.7×10⁻⁴ hPa (2×10⁻⁴ Torr) could bemaintained, the surface of the sample was irradiated with ion beams for30 seconds with an ion gun at 15 mA and 500 V. After the 30-secondirradiation, the thin-layer formation of an inorganic solid electrolytewas started by the laser abrasion method without stopping the ion-beamirradiation. More specifically, the ion-beam irradiation was performedcontinuously for 40 seconds in total. The additional 10-second periodwas overlapped with the thin-layer formation of the inorganic solidelectrolyte. The first 30-second period was for the removal of an oxidelayer, a carbonate layer, and other compound layers formed on themetallic lithium.

If the thin-layer formation is started some time after finishing theion-beam irradiation, the oxygen remaining in a vacuum container tendsto form a very thin oxide layer on the surface of the metallic lithiumwhen the thin-layer formation is performed by using an ordinary vacuumunit having an attainable vacuum degree of about 1.3×10⁻⁷ hPa (1×10⁻⁷Torr). To avoid this oxide-layer formation, the ion-beam irradiation andthe thin-layer formation were performed concurrently during the last10-second period.

After finishing the ion-beam irradiation, the thin-layer formation wascontinued. Within three minutes, the argon gas pressure was increased to2.7×10⁻³ hPa (2×10⁻³ Torr), and the temperature was raised from roomtemperature to 140° C. Under these conditions, an inorganic solidelectrolyte having a thickness of 0.5 μm was formed. The thin-layerformation was performed by using a KrF excimer laser with alaser-oscillating frequency of 5 Hz.

The above-described conditions for the ion-beam irradiation, such as anelectric current, a time period, and a gas pressure, are merely anexample. They must be adjusted in accordance with the total thickness ofan oxide layer, a carbonate layer, and other compound layers formed onthe surface of the metallic lithium, the distance between the ion gunand the sample, and other specific conditions.

If a thin inorganic solid electrolytic layer is formed by using a vacuumunit that can attain a super-high vacuum less than 1.33×10⁻⁹ hPa (1×10⁻⁹Torr) and that has an extremely small amount of adsorbed oxygen andwater vapor in the unit, a thin inorganic solid electrolytic layer maybe formed practically without forming compound layers such as an oxidelayer on the surface of the metallic lithium even when the thin-layerformation is started some time after the finishing of the ion-beamirradiation. However, the production of such a vacuum unit requiresextremely sophisticated techniques, and its price would be prohibitiveif it were produced. This is the reason why the above-described methodwas employed.

Two types of inorganic solid electrolytes were produced: Sample A wasproduced by using an Si-containing target composed of63Li₂S-36.5SiS₂-0.5Li₃PO₄ (numerals before the compounds represent themole ratio), and Sample B was produced by using an Si-free targetcomposed of 78Li₂S-21.5P₂S₅-0.5Li₃PO_(4.)

Observations of the samples after the thin-layer formation revealed thatthe thin electrolytic layer of Sample A degraded to an intense blackdiscoloration (Usually, a good Li₂S—SiS₂-based thin electrolytic layeris colorless and transparent). Its ionic conductivity was less than1×10⁻⁴ S/cm at 25° C., revealing that the direct contact with themetallic lithium caused the reaction between the Si and the metalliclithium and thereby degraded the thin solid electrolytic layer. Theionic conductivity was measured in a direction perpendicular to theplane of the layer by the DC measuring method.

On the other hand, the thin solid electrolytic layer of Sample B wascolorless and transparent, and its ionic conductivity was 1.3×10⁻³ S/cmat 25° C. The ionic conductivity was measured by the same method as inSample A. In Sample B, the ratio of the constituent Li (atom. %) was 41%when analyzed by an electron probe micro analyzer (EPMA). (Thecomposition (atom. %) of a thin electrolytic layer is usually analyzedby the foregoing EPMA, the X-ray photo-electronic spectroscope (XPS),the inductively coupled plasma (ICP) emission spectral analysis, or thegas analysis by the inert-gas fusion infrared absorption method.) Inaddition, another thin electrolytic layer was formed on a glasssubstrate under the same thin-layer-forming conditions as in Sample B.The ratio of the constituent Li, X, and the ratio of the constituent P,Y, of this layer were determined by an ICP spectral analyzer. Thecalculated ratio X/Y was 3.6. The ICP spectral analyzer used for thismeasurement was Type SPS1200VR produced by Seiko Instrument Inc. AnX-ray diffraction analysis showed only tiny peaks except the peaks fromthe substrate, confirming that Sample B was amorphous.

An analysis of the composition from the inorganic solid electrolyticlayer to the thin metallic-lithium layer was conducted by using an XPSthat was equipped with an analyzing chamber capable of maintaining asuper-high vacuum less than 1.33×10⁹ hPa (1×10⁻⁹ Torr). The XPS used forthe analysis was Type 5400 produced by Physical Electronics, Inc. Theanalysis confirmed that the oxygen content varied (decreased) graduallyfrom the electrolytic layer to the lithium layer at the boundary zonebetween the two layers. It was also confirmed that the oxide layer onthe surface of the metallic lithium was removed nearly completely beforethe formation of the inorganic solid electrolytic layer.

A negative electrode was produced by using a substrate having a thininorganic solid electrolytic layer formed on the thin metallic-lithiumlayer by using a target composed of 78Li₂S-21.5P₂S₅-0.5Li₃PO₄. A lithiumsecondary battery was produced by incorporating the negative electrode,a separator made of porous polymer film, a positive electrode, anorganic electrolysis solution, and so on. The production process andevaluation results of the battery are described below.

An electrolyte, LiPF₆, was dissolved in a mixed liquid of ethylenecarbonate (EC) and propylene carbonate (PC). The mixed liquid containingLiPF₆ was heated to further dissolve polyacrylonitrile (PAN) in highconcentration. The mixed liquid was cooled to obtain a gelatinouselectrolyte comprising LiPF₆, EC, PC, and PAN. A powder of LiCoO₂ foracting as an active material and a carbon powder giving electronicconductivity were mixed into the gelatinous electrolyte. The gelatinouselectrolyte was applied onto a sheet of aluminum foil (apositive-electrode-side collector), 20 μm in thickness, to obtain apositive electrode. The layer of the gelatinous electrolyte had athickness of 300 μm.

The negative electrode on which the thin solid electrolytic layer wasformed, a separator made of porous polymer film, and the positiveelectrode were placed in layers in a hermetically sealable stainlesssteel container. An organic electrolysis solution was produced bydissolving 1 mole % LiPF₆ as an electrolytic salt in a mixed solution ofethylene carbonate and propylene carbonate. The organic electrolysissolution was dropped into the container. The lithium secondary batterywas completed in an argon gas atmosphere having a dew point lower than−60° C.

The charge-and-discharge performance of the produced battery wasevaluated. The battery showed the following results: With a chargedvoltage of 4.2 V, when the battery was discharged at a rate of 100 mA,it showed a current capacity of 0.5 Ah (ampere hour) before the terminalvoltage decreased to 3.0 V. The energy density was 500 Wh/l (watthour/liter).

The charge-and-discharge cycle was repeated under the same condition asdescribed above. The battery was stable after more than 600 cycles. Evenafter 600 cycles, it retained more than 50% of the initial capacity.

EXAMPLE 2

A thin metallic-lithium layer having a thickness of 10 μm was formed bythe vacuum evaporation method on a sheet of copper foil having athickness of 10 μm, a length of 100 mm, and a width of 50 mm. A thininorganic solid electrolytic layer having a thickness of 0.5 μm wasformed on the surface of the thin metallic-lithium layer. As withExample 1, a sheet of metallic-lithium foil may be laminated with thesheet of copper foil.

The process of the foregoing formation of the thin electrolytic layer isexplained in detail below. The substrate having the formed thinmetallic-lithium layer was placed in a vacuum unit to treat with ionbeams. While a mixed gas of argon and nitrogen (argon: 75 vol. %,nitrogen: 25 vol. %) was fed into the unit such that a gas pressure of2.7×10⁻⁴ hPa (2×10⁻⁴ Torr) could be maintained, the surface of thesample was irradiated with ion beams with an ion gun at 15 mA and 500 V.It is desirable that the mixed gas have a nitrogen content of at least10 vol. % and at most 50 vol. %. If the nitrogen content isinsufficient, the nitriding effect is insufficient, and if the nitrogencontent is excessive, the filament of the ion gun deteriorates notably.An analysis with an XPS showed that the formed nitride layer had athickness of 1 nm.

In the above-described process, the etching treatment for removing anoxide layer and other compound layers and the nitriding treatment wereperformed concurrently by using an argon-nitrogen-mixed gas. As analternative, however, after finishing the etching treatment by usingonly an argon gas, a thin lithium nitride layer or another thin nitridelayer may be formed by the vapor deposition method. As yet anotheralternative, the formation of a nitride layer may be started in thelatter part of the ion-beam irradiation period so that the etchingtreatment by the ion-beam irradiation and the nitride-layer formationcan be performed concurrently.

Next, a thin inorganic solid electrolytic layer was formed on thenitrided thin metallic-lithium layer. The condition for forming the thinelectrolytic layer was varied to obtain various samples as shown inTables 1 and 2. When the thin-layer formation was performed at roomtemperature without heating the substrate, the sample after thethin-layer formation was heat-treated at a temperature shown in Table 2for 15 minutes in an argon gas at atmospheric pressure. The followingdata were taken on the obtained thin inorganic solid electrolyticlayers: (a) the ratio of the constituent Li (atom. %), (b) the ionicconductivity at 25° C., and (c) the ratio X/Y(expressed as an Li/P ratioin Table 1) calculated by using the ratio of the constituent Li, X, andthe ratio of the constituent P, Y. The ratio of the constituent Li wasobtained by using an EPMA. The ratio X/Y was obtained by an ICPanalysis. The ICP analysis was carried out by analyzing another thininorganic solid electrolytic layer formed on a glass or sapphiresubstrate under the same thin-layer-forming condition. These data arealso included in Tables 1 and 2. In all Samples except Sample Nos. 2—2and 2–8, the ratio of the constituent P was at least 3 atom. % and atmost 20 atom. %, and the ratio of the constituent S was at least 30atom. % and at most 60 atom. %.

TABLE 1 Material for thin inorganic solid Ratio of con-Thin-layer-forming electrolytic layer stituent Li No. method (Moleratio) (atom. %) L/P ratio 2-1  Vacuum evaporation 78Li₂S-22P₂S₅ 40 3.52-2  Vacuum evaporation 96.5Li₂S-3P₂S₅-0.5Li₃PO₄ 62 32 2-3  Vacuumevaporation 94.5Li₂S-5P₂S₅-0.5Li₃PO₄ 60 19 2-4  Vacuum evaporation85Li2_(S)-14.5P₂S₅-0.5Li₃PO₄ 48 5.9 2-5  Vacuum evaporation78Li₂S-21.5P₂S₅-0.5Li₃PO₄ 41 3.6 2-6  Vacuum evaporation65Li₂S-34.5P₂S₅-0.5Li₃PO₄ 30 1.9 2-7  Vacuum evaporation60Li₂S-39.5P₂S₅-0.5Li₃PO₄ 27 1.5 2-8  Vacuum evaporation48.0Li₂S-51.5P₂S₅-0.5Li₃PO₄ 19 0.9 2-9  Vacuum evaporation78Li₂S-21.5P₂S₅-0.5Li₃SO₄ 41 3.6 2-10 Vacuum evaporation78Li₂S-21.5P₂S₅-0.5(Li₂O-P₂O₅) 41 3.6 2-11 Vacuum evaporation78Li₂S-21.5P₂S₅-0.5Li₃PO_(3.9)N_(0.1) 41 3.6 2-12 Vacuum evaporation78Li₂S-21.5P₂S₅-0.5Li₃PO₄ 41 3.6 2-13 Vacuum evaporation78Li₂S-21.9P₂S₅-0.1Li₃PO₄ 40 3.6 2-14 Vacuum evaporation78Li₂S-21P₂S₅-1Li₃PO₄ 41 3.7 2-15 Vacuum evaporation75Li₂S-20P₂S₅-5Li₃PO₄ 43 3.8 2-16 Sputtering 78Li₂S-21.5P₂S₅-0.5Li₃PO₄41 3.6 2-17 Laser abrasion 78Li₂S-21.5P₂S₅-0.5Li₃PO₄ 41 3.6 2-18 Ionplating 78Li₂S-21.5P₂S₅-0.5Li₃PO₄ 41 3.6 2-19 Laser abrasion75Li₂S-25P₂S₅ 38 3.0 2-20 Laser abrasion 75Li₂S-25P₂S₅ 38 3.0

TABLE 2 Number of Heat treatment stable charge- Thin-layer- temperatureIonic and-discharge forming after thin-layer conductivity cycles whentemperature formation (25° C.) used in lithium No. ° C. ° C. S/cmsecondary battery 2-1  140 No heat treatment 1.3 × 10⁻³ More than 6002-2  140 No heat treatment 1.0 × 10⁻³ 450 2-3  140 No heat treatment 1.3× 10⁻³ More than 600 2-4  140 No heat treatment 1.5 × 10⁻³ More than 6002-5  140 No heat treatment 1.5 × 10⁻³ More than 600 2-6  140 No heattreatment 1.5 × 10⁻³ More than 600 2-7  140 No heat treatment 1.3 × 10⁻³More than 600 2-8  140 No heat treatment 1.0 × 10⁻³ 480 2-9  140 No heattreatment 1.5 × 10⁻³ More than 600 2-10 140 No heat treatment 1.5 × 10⁻³More than 600 2-11 140 No heat treatment 1.5 × 10⁻³ More than 600 2-12Room 150 1.5 × 10⁻³ More than 600 temperature (25° C.) 2-13 140 No heattreatment 1.4 × 10⁻³ More than 600 2-14 140 No heat treatment 1.5 × 10⁻³More than 600 2-15 140 No heat treatment 1.4 × 10⁻³ More than 600 2-16130 No heat treatment 1.5 × 10⁻³ More than 600 2-17 130 No heattreatment 1.5 × 10⁻³ More than 600 2-18 Room 150 1.5 × 10⁻³ More than600 temperature (25° C.) 2-19 170 No heat treatment 1.5 × 10⁻³ More than600 2-20 Room 170 1.4 × 10⁻³ More than 600 temperature (25° C.)

Lithium secondary batteries were produced by a method similar to thatemployed in Example 1. A negative electrode was produced by using asubstrate having a thin inorganic solid electrolytic layer formed on thethin metallic-lithium layer. The batteries were composed of the negativeelectrode, a separator made of porous polymer film, a positiveelectrode, an organic electrolysis solution, and so on.

The charge-and-discharge performances of the produced batteries wereevaluated. Each battery showed the following results: With a chargedvoltage of 4.2 V, when the battery was discharged at a rate of 100 mA,it showed a current capacity of 0.5 Ah (ampere hour) before the terminalvoltage decreased to 3.0 V. The energy density was in the range of 450to 550 Wh/l (watt hour/liter).

The charge-and-discharge cycle was repeated under the same condition asdescribed above. Excepting Sample No. 2-2, which contained an excessiveamount of Li, and Sample No. 2-8, which contained an insufficient amountof Li, the batteries were stable after more than 600 cycles. Even after600 cycles, they retained at least 50% of the initial capacity.

EXAMPLE 3

In this example, thin inorganic solid electrolytic layers were formed onthin metallic-lithium layers by using a process similar to that used inSample No. 2-5 in Example 2. Lithium secondary batteries were producedby changing the thickness of the nitride layer. The evaluation resultsof the batteries are shown in Table 3.

TABLE 3 Material for Number of stable thin inorganic Thin-layer-charge-and- Thin-layer- solid electrolytic Thickness forming dischargecycles forming layer of nitride temperature when used in lithium No.method (Mole ratio) Ion-beam treatment layer ° C. secondary battery 3-0Vacuum 78Li₂S-21.5P₂S₅- No ion-beam treatment None 140 510 evaporation0.5Li₃PO₄ 3-1 Vacuum 78Li₂S-21.5P₂S₅- Argon-ion-beam irradiation None140 590 evaporation 0.5Li₃PO₄ 3-2 Vacuum 78Li₂S-21.5P₂S₅-Argon(75%)-nitrogen(25%)-ion-  1 nm 140 More than 600 evaporation0.5Li₃PO₄ beam irradiation 3-3 Vacuum 78Li₂S-21.5P₂S₅-Argon(75%)-nitrogen(25%)-ion-  5 nm 140 More than 600 evaporation0.5Li₃PO₄ beam irradiation 3-4 Vacuum 78Li₂S-21.5P₂S₅-Argon(75%)-nitrogen(25%)-ion-  10 nm 140 More than 600 evaporation0.5Li₂PO₄ beam irradiation 3-5 Vacuum 78Li₂S-21.5P₂S₅-Argon(75%)-nitrogen(25%)-ion-  50 nm 140 570 evaporation 0.5Li₃PO₄ beamirradiation 3-6 Vacuum 78Li₂S-21.5P₂S₅- Argon(75%)-nitrogen(25%)-ion-100 nm 140 520 evaporation 0.5Li₃PO₄ beam irradiation 3-7 Vacuum78Li₂S-21.5P₂S₅- Argon(75%)-nitrogen(25%)-ion- 200 nm 140 450evaporation 0.5Li₃PO₄ beam irradiationAs can be seen from Table 3, it is desirable that the nitride layer havea thick-ness of at least 1 nm and at most 100 nm, more desirably atleast 1 nm and at most 10 nm.

Sample No. 3-1 was produced by forming the thin inorganic solidelectrolytic layer some time after finishing the argon-ion-beamirradiation for the removal of an oxide layer, a carbonate layer, andother compound layers. This thin-layer-forming method is explained inExample 1.

EXAMPLE 4

In this example, thin inorganic solid electrolytic layers were formed onthin metallic-lithium layers by using a process similar to that used inExample 2 to produce and evaluate lithium secondary batteries. In placeof the argon-nitrogen-mixed gas used in Example 2, an argon gas having apurity of 99.9999% was used for the etching treatment. After theetching, the thin inorganic solid electrolytic layers were formedwithout forming a nitride layer. The following data were taken on theobtained thin inorganic solid electrolytic layers: (a) the ratio of theconstituent Li (atom. %), (b) the ionic conductivity at 25° C., and (c)the ratio X/Y calculated by using the ratio of the constituent Li, X,and the ratio of the constituent P, Y. Tables 4 and 5 show thethin-layer-forming conditions, the ionic conductivity, and the ratio X/Y(expressed as an Li/P ratio in Table 4).

TABLE 4 Material for thin Thin-layer- inorganic solid Ratio of formingelectrolytic layer constituent Li Li/P No. method (Mole ratio) (atom. %)ratio 4-1 Laser abrasion 75Li₂S-25P₂S₅ 38 3.0 4-2 Laser abrasion75Li₂S-25P₂S₅ 37 2.9 4-3 Laser abrasion 70Li₂S-30P₂S₅ 33 2.5 4-4 Laserabrasion 70Li₂S-30P₂S₅ 34 2.6 4-5 Sputtering 80Li₂S-20P₂S₅ 42 4.0 4-6Ion plating 80Li₂S-20P₂S₅ 41 3.9 4-7 Vacuum evaporation 70Li₂S-30P₂S₅ 302.3

TABLE 5 Number of Heat treatment stable charge- Thin-layer- temperatureIonic and-discharge forming after thin-layer conductivity cycles whentemperature formation (25° C.) used in lithium No. ° C. ° C. S/cmsecondary battery 4-1 160 No heat treatment 3.0 × 10⁻⁴ More than 500 4-2Room 160 2.5 × 10⁻⁴ More than 500 temperature (25° C.) 4-3 170 No heattreatment 3.5 × 10⁻⁴ More than 500 4-4 Room 170 4.0 × 10⁻⁴ More than 500temperature (25° C.) 4-5 165 No heat treatment 2.0 × 10⁻⁴ More than 5004-6 165 No heat treatment 3.0 × 10⁻⁴ More than 500 4-7 165 No heattreatment 3.0 × 10⁻⁴ More than 500

As can be seen from Table 5, each sample had an ionic conductivity morethan 1×10⁻⁴ S/cm. An analysis of the composition from the thin inorganicsolid electrolytic layer to the thin metallic-lithium layer wasconducted by using the same XPS as used in Example 1. The analysisconfirmed that the oxygen content varied gradually from the electrolyticlayer to the lithium layer at the boundary zone between the two layers.It was confirmed that the oxide layer on the surface of the metalliclithium was removed nearly completely before the formation of the thininorganic solid electrolytic layer. A charge-and-discharge cycle testwas conducted under the same condition as in Example 2. As shown inTable 5, the batteries were stable after more than 500 cycles. Evenafter 500 cycles, they retained at least 50% of the initial capacity.

INDUSTRIAL APPLICABILITY

As explained above, the inorganic solid electrolyte of the presentinvention contains no Si and consequently can suppress the degradationof the electrolytic layer due to the reaction between metallic lithiumand silicon sulfide. In particular, the method of the present inventionforms the electrolytic layer directly on the lithium-containing metal bynearly completely removing the oxygen-containing layer on the surface ofthe lithium-containing metal. The practical absence of theoxygen-containing layer between the electrolytic layer and thelithium-containing metal enables not only the reduction in theresistance at the interface between the two members but also theaccomplishment of high ionic conductivity.

1. A method of forming an inorganic solid electrolyte by forming a thininorganic solid electrolytic layer on a substrate, the method comprisingthe step of forming a thin inorganic solid electrolytic layer comprisingLi, P, and S without containing Si while the substrate is heated at atemperature of at lowest 40° C. and at highest 180° C.
 2. A method asdefined by claim 1, wherein the inorganic solid electrolyte comprises:(a) at least 20 atom. % and at most 60 atom. % Li; and (b) the remaindercomprising P and S.
 3. A method as defined by claim 1, wherein theinorganic solid electrolytic layer is formed by a method selected fromthe group consisting of the sputtering method, the vacuum evaporationmethod, the laser abrasion method, and the ion plating method.
 4. Amethod as defined by claim 1, the method comprising the steps of: (a)etching the surface of the substrate by irradiating the surface withinert-gas ions; and (b) forming the thin inorganic solid electrolyticlayer such that at least part of the forming step is performedconcurrently with the etching step.
 5. A method as defined by claim 1,comprising forming the thin inorganic solid electrolyte layer directlyon the substrate.
 6. A method of forming an inorganic solid electrolyteby forming a thin inorganic solid electrolytic layer on a substrate, themethod comprising the steps of: (a) forming a thin inorganic solidelectrolytic layer comprising Li, P, and S without containing Si whilethe substrate is maintained at a temperature lower than 40° C.; and (b)heating the substrate having the formed electrolytic layer at atemperature of at lowest 40° C. and at highest 180° C.
 7. A method asdefined by claim 6, wherein the inorganic solid electrolyte comprises:(a) at least 20 atom. % and at most 60 atom. % Li; and (b) the remaindercomprising P and S.
 8. A method as defined by claim 6, wherein theinorganic solid electrolytic layer is formed by a method selected fromthe group consisting of the sputtering method, the vacuum evaporationmethod, the laser abrasion method, and the ion plating method.
 9. Amethod as defined by claim 6, the method comprising the steps of: (a)etching the surface of the substrate by irradiating the surface withinert-gas ions; and (b) forming the thin inorganic solid electrolyticlayer such that at least part of the forming step is performedconcurrently with the etching step.